Universidade de Aveiro - Geochemistry, petrology, and … · 2018. 1. 18. · volcanic rocks...

JME Journal of Mining & Environment, Vol.8, No.2, 2017, 139-154.

DOI: 10.22044/jme.2016.766

Geochemistry, petrology, and mineralization in volcanic rocks located in

south Neyshabour, NE Iran

A. Entezari Harsini1, S.A. Mazaheri

1*, S. Saadat

2, J.F. Santos

3

1. Department of Geology, Ferdowsi University of Mashhad, Mashhad, Iran

2. Department of Geology, Mashhad Branch, Islamic Azad University, Mashhad, Iran

3. Department of Geosciences, Geobiotec Research Unit, University of Aveiro, Aveiro, Portugal

Received 29 August 2016; received in revised form 21 October 2016; accepted 2 November 2016

*Corresponding author: [email protected] (S.A. Mazaheri).

Abstract

This paper presents the new geochemical isotopes Sr and Nd, and the mineralization data for the south

Neyshabour volcanic rocks located in NE Iran. Based on the chemical classifications, the studied rocks are

basaltic trachy andesite, trachy andesite, trachyte, and trachy dacite in composition. All the analyzed

volcanic rocks display enrichment in light rare earth elements (LREE) relative to the heavy rare earth

elements (HREE), have significant negative Ti and Nb anomalies, and have a positive U anomaly. The

tectonic discrimination diagrams for the volcanic rocks in the studied area show a post-collisional arc

environment. These characteristics are the specifications of the subduction-related volcanic rocks generated

in a post-collisional setting. The initial 87

Sr/86

Sr ratios ranging from 0.70408 to 0.70593 and the εNdi values

between +3.34 and +5 for the four samples analyzed indicate that the studied rocks are derived from a

lithospheric mantle source. Finally, it is concluded that these volcanic rocks should have formed in a post-

collisional environment that followed the Neo-Tethys subduction. There are strong evidence for copper

mineralization in these volcanic rocks. The main copper oxide minerals are malachite and atacamite. The

copper sulfide minerals such as chalcocite, minor bornite, and covellit are also present. Chalcocite is the

most abundant sulfide ore mineral present in this area. This mineralization is observed as open space filling

and thin veinlets, and it is partially controlled by linear structures and fault zones. Based on the identified

characteristics, this ore deposit is hydrothermal. Carbonate alteration is frequently seen in the area but

argillic alteration is very low, and this issue displays a hydrothermal solution with an alkaline pH.

Keywords: Chalcocite, Post-Collision Volcanic Rocks, Sr And Nd Isotopes, Neyshbour, NE Iran.

1. Introduction

The volcanic rocks in Iran are mainly classified

into three geographical categories, as follows: a

belt extended from the Maku area to the Bazman

zone, the Alborz Mountains, and the east

territories of Iran [1]. These rocks are mainly

andesitic in composition but the other types of

volcanic rocks such as rhyolite, dacite, trachyte,

and basalt\ are also present [1]. Continental

collision results in an extensive magmatic activity

not necessarily related to the widespread

extension or orogenic collapse. This activity has

been termed as the ‘post-orogenic’ [2] or

‘collision zone’ [3] magmatism. The production

of collision zone magmas is commonly linked

with lithospheric thinning [2, 4] or the mantle

upwelling following break-off of the down-going

oceanic slab [5]. Magmatism has also been

associated with dewatering of hydrous phases in

the lithospheric mantle [3, 6], melting of

deeply-subducted continental crust, and

small-scale sub-lithospheric convection [7]. The

Turkish-Iranian Plateau was formed in the Late

Cenozoic across what is now Eastern Anatolia

(Turkey), Southern Georgia, Armenia, Azerbaijan,

and Iran as a response to the Arabia-Eurasia

collision. The region is a prime site for the study

Entezari Harsini et al./ Journal of Mining & Environment, Vol.8, No.2, 2017

140

of collision magmatism, being a young part of the

Alpine–Himalayan belt with abundant and

reasonably well-studied volcanic and intrusive

rocks. There is also an ever-improving

geophysical dataset that can be used to link

magmatism to the geodynamic processes [8-10].

Most collision magmatism in the plateau has an

incompatible element-enriched calc-alkaline or

shoshonitic signature with trace element evidence

for the presence of subduction-modified

lithosphericmantle sources [4, 11-12]. Magmatic

activity has been variously linked to the processes

above [3, 4, 6, 7, and 12]. The studied area is

located 220 km SW of Mashhad (capital of the

Khorasan Razavi province in NE of Iran) and 80

km south of Neyshabour (a city in the Razavi

Khorasan province). The area extends from 58°

42' 15" to 58° 43' 40" east longitude and 35° 47'

30" to 35° 52' 00" north latitudes, equivalent to 48

km2 (Figure 1). This region is located in the NE of

the central Iran zone and inside the Sabzevar

structural zone north of the Daruneh fault [13,

14-15]. There are some chromite ore deposits in

the ophiolitic rocks in the Sabzevar structural

zone. In addition, volcanic and intrusive rocks that

have large outcrops in this area indicate

mineralization of copper, copper-gold (such as

Abbasabad region), gold (such as Koohzar

Damghan and Gandi regions), and

copper-lead-zinc mineralization. This paper

presents new data for the mineralization and

petrochemical characteristics of the Sr and Nd

isotopes, which can provide a mineralization and

petrogenetic model from magmatism in this area.

2. Method

2.1. Preparation of samples

More than 183 thin sections, 10 thin polished

sections, and 10 polished blocks were prepared in

the laboratory of the Ferdowsi University of

Mashhad for the petrography, mineralization, and

alteration studies. The samples were collected

from the surface. The location of samples

presented in Figure 2.

2.2. Major and trace element geochemistry

According to the petrographic studies, twenty

samples of unaltered rocks were selected for

chemical analysis. The selected samples were

crushed and powdered using mild steel, and sent

to Kansaran Binaloud Company (Tehran, Iran) for

measuring the major elements by XRF. Moreover,

the samples were sent to Acme laboratories

(Vancouver, Canada) for trace element analysis

by ICP mass spectrometry, which was carried out

based on the lithium metaborate-tetraborate fusion

and nitric acid digestion.

Figure 1. Location of studied area in NE Iran.


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Figure 2. Location of samples in rock units from south Neyshabour area.

2.3. Sr and Nd isotopic analysis

Based on the petrographic studies and

geochemical information, four samples from the

volcanic rocks with low LOI and minimum

alteration were selected and sent to the Laboratory

of Isotope Geology of the Aveiro University

(Portugal) for determination of the Sr and Nd

isotope compositions.

3. Results

3.1. Petrography

Based on the petrographical observations, the

studied volcanic rocks indicate porphyritic

textures with phenocrysts of plagioclase,

K-feldespar, opacity hornblende, pyroxene, and

magnetite, embedded in a fine-to-medium-grained

groundmass. Moreover, the amygdaloidal and

poikilitic textures were seen in thin sections

(Figure 3). Based on the assemblage of the

minerals and their percentage, the volcanic rocks

of this area can be divided into 9 groups,

including: Hornblende pyroxene olivine andesite

basalt, Olivine hornblende andesite basalt,

Pyroxene hornblende trachy andesite, Hornblende

pyroxene trachy andesite, Hornblende andesite,

Andesite, Hornblende trachy andesite, Trachy

andesite and Pyroxene andesite (Figure 4.)

Figure 3. Photomicrographs of thin sections from volcanic rocks in studied area, XPL: (a) Plagioclase and

pyroxene phenocrysts in microlitic groundmass (b) Plagioclase and olivine phenocrysts in fine-grained

groundmass (c) K-feldspar, hornblende, magnetite phenocrysts, and glomeroporphyritic texture (Pl=Plagioclase;

Cpx=Clinopyroxene; Ol=Olivine; Kfs=K-feldspar; Hbl=Hornblende; Mag=Magnetite).


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Figure 4. Geological map of studied area.

3.2. Major, trace, and rare earth elements;

geochemistry

Table 1 indicates the symbols and locations of the

samples. The results obtained for the major and

trace elements for 20 samples of no to

slightly-altered volcanic rocks are presented in

Table 2. The total alkali-silica (TAS)

classification diagram [16] is used to categorize

the volcanic rocks. These samples were plotted

within the basaltic trachy andesite, trachy

andesite, trachyte, and trachy dacite fields (Figure

5a). Most of the samples were located in the

alkaline section and boundary of alkaline and

sub-alkaline field [17] (Figure 5b), and their

geochemical character with alumina saturation

index [ASI = molar Al2O3/(CaO + K2O + Na2O)]

is ranged from 0.71 to 1.02. Using SiO2 as a

fractionation index, the MgO, FeO, and CaO

contents displayed negative correlation with SiO2

content, while the Al2O3, K2O, and Na2O contents

represented positive correlation with SiO2 content;

moreover, TiO2 and P2O5 were dispersed, and they

showed no correlation with the SiO2 content

(Figure 6 a-h). The linear trends among SiO2 and

several major oxides demonstrates a genetic

relationship in these volcanic rocks. Along with

increasing the silica content, CaO, MgO, and FeO

decreased in these rocks. Probably such trends are

compatible with fractionation of the phenocryst

phases observed, which were derived from a

basaltic andesite as a parental magma. In fact, this

type of variation displays that Mg- and Fe-rich


143

rock-forming minerals contain amphibole,

pyroxene, and olivine, so they have played a great

role during crystallization of the volcanic rocks

(mafic minerals like olivine and pyroxene are

crystallized early in the process that results in

removing Mg, Fe, and Ca, and enrichment of

liquid in silica) like plagioclases, and represents

the effect of fractional crystallization on a pending

magmatic evolution. As plagioclase forms a

continuous solid solution between albite

(NaAlSi3O8) and anorthite (CaAl2Si2O8), the

crystallization temperature changed. Anorthite has

a higher crystallization temperature, so the early

formed plagioclase crystals are Ca-rich. As Ca is

used up in the melt, the crystallizing plagioclase

becomes progressively more Na-rich. Na2O,

Al2O3, and K2O vs. SiO2 diagrams display a

positive trend. This issue points out that

K-feldspar and sodic plagioclases (Na-rich

plagioclase) were crystallized in the late stage of

the crystallization and fractionation of a silicate

melt in low pressures. The increasing amount of

Al2O3 is due to their more participation in

feldspars. During the fractional crystallization of

the magma, the incompatible elements such as the

La, Nb, Zr, Rb, Th, Ta, and Hf contents increased

with increasing SiO2 concentrations (Figure 6 i-o).

The incompatible trace element variations in these

igneous rocks are in agreement with plagioclase

and amphibole [18]. As a result, the crystal

fractionation process is specified from the parental

magma. The trace element of the samples was

normalized to primitive mantle [19] (Figure 7a),

and indicate that the volcanic rocks are depleted in

high field strength elements (HFSE), which may

be owing to fractionation of a titanium-rich phase

in the magma source [20]. Additionally, all

samples have enrichment in the large ion

lithophile (LILE). The above-mentioned

characteristics point out that the volcanic rocks

would belong to the subduction zones. Figure 6b

shows a range of chondrite-normalized

distribution REE patterns [21] for 20 analyzed

samples. The LaN/SmN ratios vary between 2.35

and 4.09 (on average, 3.37), which display

moderate enrichment in light rare earth elements

(LREE). Eu anomaly is calculated by the

Eu/Eu*=EuN/(SmN×GdN)1/2

formula (Taylor and

McLennan, 1985). The Eu anomaly values range

from 0.72 to 0.97 (on average, 0.87), which

represent weakly negative Eu anomaly [22] for

the studied samples, and the GdN/YbN values

range from 1.41 to 1.86 (on average, 1.65). These

ratios reflect nearly flat heavy rare earth element

(HREE) on the normalized REE patterns

compared to the chondrite values [23].

Table1. Symbol and location of samples (UTM) of south Neyshabour volcanic rocks.

Symbol Zone Lat Long Sample

S-119 656119 3962448 40S S-154 653057 3968425 40S S-131 653768 3965241 40S S-179 652671 3965670 40S S-144 655354 398326 40S S- 60 654773 3966669 40S S-17 655352 3966494 40S

S-113 655634 3964537 40S S-140 654365 3968353 40S S-35 654994 3966895 40S

S-160 652970 3966897 40S S-183 655638 3965998 40S S-158 652970 3966879 40S S-157 654126 3966522 40S S-16 655416 3966508 40S S-71 655737 3966255 40S S- 40 655269 3966919 40S S- 31 655092 3967049 40S S-57 654632 3967377 40S S- 86 655994 3965417 40S S-171 657228 3964983 40S S-92 657265 3964785 40S

https://en.wikipedia.org/wiki/Fractional_crystallization_%28geology%29

https://en.wikipedia.org/wiki/Magma


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Table 2. Major, trace, and rare earth element analysis of south Neyshabour volcanic rocks.

Sample S-171 S-92 S-119 S-153 S-131 S-179 S-60 S-17 S-140 S-35 S-160 S-183 S-16 S-71 S-57 S-86 S-40 S-31 S-27 S-49

SiO2 62.58 61.54 54.01 54.39 55.01 54.56 57.86 57.12 57.71 57.35 58.01 57.25 57.86 57.42 60.88 59.35 58.36 58.12 55.87 55.12

Al2O3 17.69 17.59 15.91 16.05 15.86 15.62 17.99 18.03 18.24 18.35 18.02 18.42 17.08 16.89 17.52 18.13 17.63 17.51 15.24 15.35

FeO 3.50 3.82 8.21 7.92 7.53 7.79 4.80 5.35 5.19 5.12 5.23 5.17 5.21 5.34 3.73 4.22 4.85 5.01 4.26 4.78

CaO 5.27 5.92 6.87 6.52 7.02 7.32 4.26 4.62 5.17 5.21 5.24 5.27 2.86 3.42 1.68 1.89 3.01 2.97 4.98 5.02

Na2O 4.48 4.53 3.56 3.45 3.29 3.35 4.34 4.12 3.94 4.25 3.59 3.75 4.75 4.35 4.26 4.12 3.87 3.67 2.98 2.87

K2O 3.02 2.95 2.99 3.25 3.38 3.57 5.49 5.15 4.02 3.98 4.08 3.92 6.89 6.72 7.34 6.95 7.02 6.99 7.41 7.25

MgO 0.27 0.42 5.24 5.10 4.52 4.98 1.78 1.98 2.35 2.37 2.34 2.52 1.69 1.91 1.48 1.79 1.49 1.78 0.98 1.35

Cu 0.02 0.02 0.01 0.01 0.01 0.01 0.02 0.02 0.01 0.01 0.01 0.01 0.05 0.07 0.05 0.06 0.10 0.06 1.69 1.81

TiO2 1.429 1.354 0.891 0.854 0.840 0.915 0.968 1.071 0.869 0.864 0.867 0.862 1.011 1.009 0.914 1.050 0.918 0.899 1.154 1.034

MnO 0.080 0.069 0.112 0.131 0.098 0.082 0.069 0.059 0.045 0.048 0.042 0.046 0.124 0.105 0.040 0.051 0.078 0.087 0.063 0.087

P2O5 0.336 0.392 0.341 0.313 0.362 0.333 0.454 0.521 0.418 0.455 0.418 0.428 0.732 0.792 0.323 0.355 0.513 0.532 0.468 0.423

S 0.009 0.008 0.054 0.057 0.009 0.008 0.012 0.013 0.012 0.015 0.008 0.009 0.018 0.035 0.011 0.019 0.008 0.009 0.017 0.017

L.O.I 1.04 1.15 1.62 1.78 1.87 1.55 1.73 1.76 1.66 1.67 1.85 1.87 1.43 1.66 1.51 1.72 2.03 2.10 4.40 4.65

Total 99.72 99.86 99.82 99.83 99.80 99.71 99.77 99.81 99.64 99.52 99.71 99.53 99.71 99.72 99.74 99.71 99.87 99.76 99.51 99.76

Ba 287 294 383 367 376 381 515 552 482 475 451 453 551 525 485 435 570 552 497 505

Be 2 2 1 2 1 1 2 2 1 2 2 2 1 1 2 1 2 2 4 3

Co 4.3 5.6 27.4 28.2 28.7 29.1 9.8 10.1 12.1 12.8 12.7 13.1 8.0 9.1 4.8 5.2 7.4 6.9 6.1 5.8

Cs 1.1 1.5 1.9 2.4 2.2 2.6 2.9 3.1 1.9 1.5 1.3 1.2 3.3 2.8 3.6 3.2 3.3 2.9 3.0 2.8

Ga 19.6 18.3 16.1 15.9 15.6 16.2 15.1 15.6 18.2 18.8 17.2 16.8 15.5 16.1 16.0 15.5 15.0 14.3 13.7 14.1

Hf 7.5 7.4 2.9 2.8 2.9 2.9 3.7 3.6 3.3 3.5 3.5 3.4 3.6 3.7 4.1 4.2 3.7 3.8 3.4 3.3

Nb 27.9 26.2 9.9 10.4 11.5 10.8 15.2 15.8 14.5 14.1 14.3 14.6 16.5 16.2 19.2 18.8 15.8 15.6 15.3 15.7

Rb 68.0 73.1 70.8 72.4 74.4 79.1 135.4 142.1 97.0 110.2 96.1 82.2 160.4 155.2 182.2 173.1 158.7 148.9 144.9 145.5

Sn 3 2 2 1 1 2 1 1 1 2 1 1 1 1 1 2 2 1 1 1

Sr 453.9 438.2 614.8 580.3 559.8 592.2 608.6 595.1 726.7 717.3 728.7 731.2 504.7 491.2 239.1 251.2 454.3 432.1 347.2 371.1

Ta 1.8 1.6 0.6 0.7 0.7 0.8 0.9 0.8 0.8 0.9 1.0 0.8 1.1 1.0 1.2 1.1 0.9 1.0 1.0 0.9

Th 9.1 9.5 3.7 4.3 4.1 4.8 6.1 6.2 4.9 5.1 5.0 4.8 7.2 6.9 7.8 7.4 6.7 7.1 6.2 6.3

Nb 2.7 2.5 1.1 1.3 1.1 1.2 1.6 1.4 1.4 1.6 1.5 1.4 2.3 2.5 1.1 1.1 2.3 2.4 2.4 2.1

V 112 120 259 262 252 243 298 310 265 256 269 285 395 375 223 259 361 345 279 242

W 1.4 1.6 1.5 1.9 2.9 3.1 1.3 1.7 1.5 1.8 1.7 1.5 1.4 1.6 2.0 1.8 1.8 1.6 3.3 2.7

Zr 306.1 297.3 108.6 121.5 119.8 109.1 157.9 149.1 142.0 138.5 140.9 135.2 167.5 159.5 193.5 185.1 160.0 163.6 148.0 152.1

Y 30.8 31.2 19.3 18.9 19.4 18.8 16.9 17.1 17.1 16.8 17.9 17.2 18.6 18.3 18.0 18.5 17.4 17.1 16.7 17.2


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Table 2. Continued

Sample S-171 S-92 S-119 S-153 S-131 S-179 S-60 S-17 S-140 S-35 S-160 S-183 S-16 S-71 S-57 S-86 S-40 S-31 S-27 S-49

La 36.3 35.2 15.4 15.8 17.1 16.8 22.6 21.7 21.3 21.5 20.9 21.2 24.7 24.1 24.1 23.2 24.1 23.8 23.3 23.9

Ce 76.9 75.2 32.0 33.2 36.0 35.2 42.6 43.2 43.6 41.9 41.6 42.3 48.3 47.9 45.6 44.9 45.7 45.2 45.8 44.9

Pr 8.96 8.52 4.35 4.71 4.62 4.44 5.25 5.37 5.24 5.01 5.14 4.93 5.71 5.54 5.36 5.59 5.72 5.45 5.51 4.89

Nd 35.8 36.4 17.7 18.1 18.6 18.4 19.9 20.2 19.2 18.8 20.2 20.4 22.3 22.6 20.6 19.9 21.4 22.3 22.3 22.5

Sm 7.14 7.01 4.12 3.93 3.88 4.07 3.75 3.83 4.11 3.95 3.99 4.05 4.39 4.09 3.71 3.91 3.86 4.01 4.00 3.89

Eu 1.69 1.58 1.22 1.17 1.15 1.11 1.05 0.98 1.15 1.19 1.15 1.13 1.11 1.15 0.91 1.08 1.18 1.21 1.09 1.12

Gd 6.69 6.35 4.12 3.93 4.22 4.17 3.53 3.61 3.73 3.68 3.80 3.75 3.67 3.86 3.33 3.52 3.49 3.62 3.57 3.49

Tb 1.04 1.11 0.61 0.56 0.65 0.61 0.57 0.59 0.58 0.58 0.59 0.60 0.60 0.63 0.51 0.56 0.58 0.57 0.54 0.55

Dy 5.64 5.23 3.44 3.61 3.74 3.54 2.91 3.11 3.36 3.21 3.23 3.39 3.51 3.33 2.97 3.08 3.32 3.45 3.01 2.96

Ho 1.10 1.05 0.74 0.72 0.75 0.73 0.60 0.62 0.73 0.68 0.69 0.72 0.65 0.63 0.65 0.69 0.63 0.66 0.61 0.67

Er 3.20 3.11 1.81 1.87 1.99 1.92 1.78 1.81 1.90 1.94 1.92 1.95 1.88 1.83 1.90 1.94 2.06 1.98 1.79 1.85

Tm 0.44 0.41 0.30 0.29 0.30 0.30 0.27 0.28 0.27 0.28 0.29 0.29 0.29 0.29 0.29 0.28 0.28 0.27 0.30 0.30

Yb 2.91 2.76 1.95 1.91 1.92 1.89 1.70 1.72 1.84 1.86 1.73 1.87 1.87 1.92 1.91 1.88 1.84 1.87 1.71 1.73

Lu 0.46 0.48 0.31 0.30 0.31 0.31 0.27 0.28 0.29 0.27 0.28 0.29 0.32 0.30 0.31 0.30 0.31 0.31 0.28 0.29

Nb/Th 3.07 2.757 2.68 2.42 2.80 2.25 2.49 2.54 2.96 2.76 2.86 3.04 2.29 2.35 2.46 2.54 2.36 2.2 2.47 2.49

Ba/Sr 0.63 0.67 0.62 0.63 0.67 0.64 0.85 0.93 0.66 0.66 0.62 0.62 1.09 1.06 2.02 1.73 1.25 1.27 1.43 1.36

Th/Yb 3.13 3.44 1.90 2.25 2.14 2.54 3.59 3.6 2.66 2.74 2.89 2.57 3.85 3.59 4.08 3.94 3.64 3.8 3.63 3.64

La/Nb 1.30 1.34 1.56 1.52 1.49 1.56 1.49 1.37 1.47 1.52 1.46 1.45 1.50 1.49 1.25 1.23 1.53 1.53 1.52 1.52

(La/Sm)N 3.20 3.16 2.35 2.53 2.77 2.60 3.79 3.56 3.26 3.42 3.29 3.29 3.54 3.71 4.09 3.73 3.93 3.73 3.66 3.86

Eu/Eu* 0.75 0.72 0.91 0.91 0.87 0.82 0.88 0.81 0.90 0.95 0.90 0.89 0.85 0.88 0.79 0.89 0.98 0.97 0.88 0.93

(Gd/Yb)N 1.85 1.86 1.70 1.66 1.77 1.78 1.68 1.69 1.64 1.60 1.77 1.62 1.58 1.62 1.41 1.51 1.53 1.56 1.64 1.63


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3.3. Sr and Nd isotopes

The Sr and Nd isotopic data obtained in the four

selected samples are presented in Table 3. These

samples have the 143

Nd/144

Nd and 87

Sr/86

Sr ratios

ranging from 0.512807 to 0.512893 and from

0.704082 to 0.705931, respectively. If the Nd

isotopic ratio composition is expressed using the

εNd notation, the range goes from +3.3 to +5.0.

For Sr and Nd, the isotopic ratios and εNd values

show insignificant differences between the

present-day values and the calculated initial

values for young geological ages like those

obtained in this work (less than 6 Ma) since both 87

Rb and 147

Sm have very low decay constants

[24, 25]. Therefore, the measured isotopic ratios,

in this case, may be considered, in practical terms,

as representing the initial compositions.

Figure 5. (a) Na2O + K2O vs. SiO2 diagram for classification of volcanic rocks [16]; (b) Na2O + K2O vs. SiO2

diagram to distinguish between subalkaline and alkaline fields [17].


147

Figure 6. Selected major and trace elements vs. SiO2 contents for the south Neyshabour volcanic rocks.


148

Figure 7. (a) Primitive-mantle normalized diagrams [30]; (b) REE chondrite normalized [32].

Table 3. Rb–Sr and Sm–Nd isotopic data for south Neyshabour volcanic rocks.

Sample Rb

(ppm)

Sr

(ppm) 87Rb/86Sr 87Sr/86Sr

error

(2σ) Sm(ppm) Nd(ppm) 147Sm/144Nd 143Nd/144Nd

error

(2σ) εNd

S- 16 106 505 0.609 0.705047 0.000020 4.39 22.3 0.119 0.512833 0.000017 +3.8

S- 57 182 239 2.205 0.705931 0.000020 3.71 20.6 0.109 0.512807 0.000015 +3.3

S-171 68.0 454 0.433 0.704082 0.000021 7.14 35.8 0.121 0.512893 0.000015 +5.0

S-119 70.8 615 0.333 0.704614 0.000016 4.12 17.7 0.141 0.512843 0.000017 +4.0

4. Discussion

4.1. Source properties

The negative and positive εNd(t) isotopic ratios

demonstrate the features of the crustal melt and

mantle array, respectively [26, 27]. With respect

to the εNdi–(87

Sr/86

Sr)i isotopic data depicted in

Figure 8, the south Neyshabour volcanic rocks

show a trend toward the mantel [26]. These

samples are mainly characterized with high

concentrations of the incompatible trace elements

(e.g. LILE and LREE). Moreover, Figure 5 also

shows that these samples are marked by

considerable negative Nb, Ta, and Ti anomalies,

specifying that the samples have not been driven

from normal MORB or OIB mantle sources,

which usually demonstrate positive Nb-Ti

anomalies in primitive-mantle normalized trace

element diagrams [28]. Moreover, the Nb/La

ratios of the south Neyshabour volcanic samples

were changed from 0.64 to 0.79, representing a

lithospheric mantle source [11, 29]. Thus these

volcanic rocks have possibly been derived from

an enriched mantle source containing a distinct

subduction signature. The trace-element

compositions of the south Neyshabour volcanic

rocks (NE Iran) also indicate that their parental

magma was produced by generally low degrees of

partial melting. The (La/Yb)N value for these

samples varied from 5.32 to 9.31. This ratio was

less than the (La /Yb)N ratio in the igneous rocks,


149

which have been derived from the magma with

garnet ((La/Yb)N > 20) as the major phase in their

source [6, 11, 30], representing the garnet as the

major residual phase. Therefore, spinel and/or

amphibole may exist in the residua phase.

Nevertheless, the slight enrichment in MREE of

the samples (Figure 7) indicates that amphibole

may not stay stable in the residual following

fractional melting [30, 31]. La/Yb versus Yb is

also useful to assess the degree of mantle partial

melting and distinguish between melting in the

spinel and garnet fields [31-33]. There is a very

small change in the La/Yb ratio in the

spinel-facies melts compared with their mantle

source. In contrast, there are large changes in Yb

in the melts formed in garnet-facies mantle [31,

33]. Based on the La/Yb ratio and the amount of

Yb of south Neyshabour volcanic rocks, neither a

variables degree of partial melting in a spinel

lherzolite nor a variables degree of partial melting

of garnet lherzolite solely can generate the

observed variation in the La/Yb ratio by changing

Yb. One possible explanation for the La/Yb

versus Yb compositions of these volcanic rocks

involves mixing of small melt fraction from

garnet-facies mantle with a relatively larger melt

fractions from the spinel-facies mantle.

Figure 8. Plot of initial (

87Sr/

86Sr) i vs.

εNd ratios.

4.2. Tectonic setting

Application of TiO2 vs. Al2O3 [34] indicates that

all samples are placed in the arc environment

(Figure 9a). Plotting the samples on the

Nb * 50-Zr*/3-Ce/P2O5 ternary diagram [34]

shows that the south Neyshabour volcanic rocks

are situated at post-collisional arc (Figure 9b).

Researchers [e.g. 35] have used the Nb/Th ratio to

survey the effect of subduction zone processes on

the formation of the volcanic rocks. This ratio is

lower than ‘4’ in the rocks related to the

subduction zone [19]. The Nb/Th ratio of the

proposed samples changed from 2.19 to 3.06. As a

result, the Nb/Th ratio in the proposed samples is

less than ‘4’, and this verifies that the studied

rocks were affected by the processes related to the

subduction zone. As mentioned earlier, the studied

area is located in the Sabzevar zone (NE Iran).

During the Late Cretaceous, the Sabzevar oceanic

basin (branch of the Neo-Tethys Ocean between

central Iranian micro-continent and Eurasian

margin) was formed, and then it closed during

Paleocene-Eocene [36]. With regard to the

geochemical properties and tectonic environment

of the south Neyshabour volcanic rocks, we

believe that they were created in the post-collision

environment after the closure of this branch of

Neo-Tethys. Enrichment in LILE and LREE

relative to HFSE and HREE, respectively, and

negative anomalies of the Nb and Ti elements are

the specifications of the subduction-related

magmas. These characteristics are generally

related to a mantle source, which has already been

enriched in LILE and LREE over HFSE and

HREE by the metasomatic activity of fluids

released from the subducted sediments or oceanic

slab [37, 38]. Many different types of models have

been suggested to illustrate this signature in the


150

post-collisional setting such as considering

amphibolites, peridotite, and metapelite as a

hybrid source in the crust–mantle boundary [39]

or presenting a theory regarding mixing of

magmas in the asthenosphere and enriched

lithospheric mantle [40]. Considering all

evidence, the best suggestion can be an

enrichment of the lithospheric mantle by earlier

subduction events in a post-collision stage [41].

Figure 9. (a) TiO2 vs. Al2O3 binary diagram to separate within plate environment and arc related, [34]; (b)

Nb*50-Zr*/3-Ce/P2O5 ternary diagram to separate continental arc and post-collisional arc [34].

4.3. Mineralization

Carbonate and chlorite minerals are of malachite

and atacamite, respectively, and sulfide mineral of

chalcocite, minor bornite, and covolite are seen on

the surface of the studied volcanic rocks. Copper

minerals are seen in association with volcanic

rocks at all sections of the area; these minerals

have mainly filled the thin veinlets and voids.

Figure 10 shows this issue. Mineralogical findings

were checked by the XRD test (Figure 11). In

XRD, the results were discriminated minerals of

malachite, atacamite, and chalcocite but covolite

and bornite were not detected because of their

small amount in the samples. The main sulfide ore

mineral in this region is chalcocite. Enormous

amounts of chalcocite, not a high amount of Fe

oxide and bornite, in the region show that

chalcosite is mainly from the first generation, and

in the second generation, it is lower than what

probably formed from the conversion of bornite. There is also the possibility that large quantities of

chalcocites changed into malachite and atacamite

under the oxidizing conditions by meteoric

waters. However, copper minerals were formed in

two phases. The first mineralization phase was

calchocite with minor bornite, and the secondary

phases had more malachite and atacamite and low

chalcocite and covellite (Figure 12). The suitable

space for transferring mineralizer solutions is

provided by pores in rocks and rock joints, which

have cut the host rocks in different directions. In

addition, the mineralization is seen as leakage and

replacement in some places. In some parts of the

region, there are certain evidence that

copper-bearing solutions are controlled by linear

structures and fault zones. According to the

composition and texture of the wall rocks and

solution volume, this feature occurred from a few

centimeters to several meters in the host rock. In

the near of the faults and contacts between two

units of rocks, large amounts of mineralization

and carbonate alteration was observed, and this

alteration shows an alkaline pH. The primary

chalcosite is formed in low sulfur and Eh

conditions. Pyrite mineralization is absent in the

studied area, and this issue represents that the

amount of iron is low in the mineralizer solutions.

Along with get away from the faults and moving

toward the upper rock units, the mineralization

evidence disappeared or limited. Based on the

identified characteristics, this deposit is a

hydrothermal ore deposit whose solutions have

alkaline pH and low iron, sulfide, and Eh. Copper

sulfide mineralization depends on the parent

magma, which has analkaline nature with good

differentiation. In the fluid phase, because of the

alkaline nature, its residue has many Na and Ca

ions with ore-forming cations, so their derived

solutions have high pH. Based on the petrographic

studies, the south Neyshabour volcanic rocks have

a high magnetite but they do not show the origin

of mineralization if they are the origin of

mineralization. For a high Fe in magma, we

should see pyrite in the mineralization zone but

the absence of pyrite shows that we need drilling

at depth for finding its origin.


151

Figure 10. (a) Chalcocite into veinlet is converted to covollite, PPL; (b) Chalcocite into void,PPL; (c) Malachite

into veinlet, XPL; (d) Malachite into voids, PPL. (Cc= Chalcocite; Cv= Covollite; Mlc= Malachite).

Figure11. Results of XRD test.


152

Figure 12. Paragenetic sequence of copper mineralization in studied area.

5. Conclusions

Based on the total alkali-silica (TAS)

classification diagram, the volcanic rocks in the

studied area are basaltic trachy andesite, trachy

andesite, trachyte, and trachy dacite; they have

been formed from the same magma by the crystal

fractionation process. In primitive mantle and

chondrite-normalized trace element spider

diagrams, the analyzed samples demonstrate

slight-to-moderate enrichment in LILE (2.35 ≤ La

N / SmN ≤ 4.09) compared with HFSE (1.41≤Gd N

/YbN≤ 1.86), while being accompanied by the

negative anomalies of Nb and Ti. These

geochemical properties and the isotope

geochemistry (Sri and εNdi) illustrate that the

studied rocks are derived from a metasomatized

lithospheric mantle source. The analyzed samples

show a post-collisional arc environment. We

suggest that the volcanic activity in the studied

area after Eocene period has been created by the

enriched lithospheric mantle source, which has

previously been metasomatized by subducted

sediments after Neo-Tethys subduction. Based on

the field and laboratory studies, mineralization in

this region occurred in the suitable tectonic setting

related andesitic units, and the active tectonic

plates caused the formation of joint systems,

cracks, veins, veinlets, and fractures in rocks.

These have turned into channels for the

infiltration of copper-bearing solutions from the

depths to the surface resulting in the expansion of

copper mineralization in the region. Minerals of

malachaite, atacamite, chalcosite, and minor

bornite and covollite are observed in this area.

Based on our studies, mineralization in this area is

of hydrothermal type, and the sulfide minerals

have been formed under reduction conditions, low

sulfur, and alkaline PH, and finding its origin

requires drilling in depth.

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5931م، سال ودم، شماره تشهدوره زیست، پژوهشی معدن و محیط -و همکاران/ نشریه علمیهرسینی انتظاری

ی ولکانیکی جنوب نیشابور، شمال شرق ایرانها سنگزایی ژئوشیمی، پترولوژی و کانی

3و ژوزه فرانسیسکو سنتوز 2سعید سعادت ،1*سید احمد مظاهری ،1اعظم انتظاری هرسینی

دانشگاه فردوسی مشهد، ایران شناسی، زمینگروه -1

ن، دانشگاه آزاد اسلامی واحد مشهد، ایراشناسی زمینگروه -2

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[email protected]* نویسنده مسئول مکاتبات:

چکیده:

نیشابور در شمال جنوب فشانی آتش های سنگزایی استرانسیوم و نئودیوم و همچنین کانه یها زوتوپیاجدیدی را در خصوص ژئوشیمی، های داده تحقیقاین

و تراکی آندزیت بازالت، تراکی آندزیت، تراکیت های محدودهمنطقه مورد مطالعه در فشانی آتش های سنگمیایی یش بندی طبقه. بر اساس دهد میشرق ایران ارائه

عناصر کمیاب شدگی غنی دهنده نشانها کندریت نسبت به ها آندن و نرمالیزه کر فشانی آتش های این سنگ. تجزیه ژئوشیمیایی گیرند میتراکی داسیت قرار

در توان می مثبت نسبت به اورانیوم را هنجاری بیمنفی از عناصر نئوبیوم و تیتانیوم و هنجاری بیهمچنین است وسبک نسبت به عناصر کمیاب سنگین

ها سنگ. نمودارهای زمین ساختی خاستگاه این باشند می مناطق فرورانشمتعلق به ها ویژگید که این ال شده به گوشته اولیه مشاهده کرنمودارهای عنکبوتی نرم

های مورد از سنگ برای چهار نمونه +1+ تا 94/9 از εNdi و مقدار 811339/1تا 81418/1از 87Sr/86Sr. مقدار اولیه کند میرا کمان پس از برخورد معرفی

های سنگبه این نتیجه رسید که توان میاست. در نهایت یا گوشته منشأبیانگر این مقادیر که اشندب متغیر میکه مورد تجزیه ایزوتوپی قرار گرفتند مطالعه

های سنگسازی زیادی را در . آثار کانیاند شدهمنطقه مورد مطالعه در یک محیط پس از برخورد به دنبال فرورانش اقیانوس تتیس جوان تشکیل فشانی آتش

اولیه های یکانثانویه مالاکیت، آتاکامیت و کوولیت و های یکانبه قابل مشاهده در منطقه های یکاند. از کرمشاهده توان میمنطقه مورد مطالعه فشانی آتش

ار پر کننده فضای خالی و به مقدرگچه، -سازی بیشتر به صورت رگهنید. کاکراشاره توان میبورنیت یجزئو به مقدار کالکوسیت به مقدار فراوانسولفید مس،

توسط ساختارهای ها آن دهنده یلتشک یها محلولتیک بوده و حرکت ژنسازی اپی کمتر به صورت پراکنده در متن سنگ قرار دارد آنچه مشخص است که کانی

فشانی آتش های سنگ. به مقدار فراوان دگرسانی کربناته در استسازی از نوع هیدروترمال کانی دهنده نشاناین خصوصیات و شود یمکنترل ها گسلخطی و

با محلول هیدروترمالی کانی زایی اولیه منشأدهد که نشان می مسئلهکه این شود ینمو آثار دگرسانی آرژلیک در منطقه مشاهده است قابل مشاهده مورد مطالعه

pH .قلیایی است

سترانسیوم، نیشابور، شمال شرق ایران.ی نئودیوم و اها زوتوپیافشانی بعد از برخورد، های آتش کالکوسیت، سنگ کلمات کلیدی:

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